declare a resValue in your defaultConfig which will become the Application's name. (Attention: if you choose to name it app_name, be sure to delete the original app_name property from your strings.xml file, or it will clash.)

set your productFlavors as you did already. You can leave them empty if it is ok for you to concat your App's name with the flavor's name only, or provide an explicit resValue, which will override the value from defaultConfig.